Ethylbenzene (EB), para-xylene (PX), ortho-xylene (OX) and meta-xylene (MX) are often present together in C8 aromatic product streams from chemical plants and oil refineries. Although high purity EB is an important raw material for the production of styrene, for a variety of reasons all high purity EB feedstocks used in styrene production are produced by alkylation of benzene with ethylene, rather than by recovery from a C8 aromatics stream. Of the three xylene isomers, PX has the largest commercial market and is used primarily for manufacturing terephthalic acid and terephthalate esters for use in the production of various polymers such as poly(ethylene terephthalate), poly(propylene terephthalate), and poly(butene terephthalate). While OX and MX are useful as solvents and raw materials for making products such as phthalic anhydride and isophthalic acid, market demand for OX and MX and their downstream derivatives is much smaller than that for PX.
Given the higher demand for PX as compared with its other isomers, there is significant commercial interest in maximizing PX production from any given source of C8 aromatic materials. However, there are a number of major technical challenges to be overcome in achieving this goal of maximizing PX yield. For example, the four C8 aromatic compounds, particularly the three xylene isomers, are usually present in concentrations dictated by the thermodynamics of production of the C8 aromatic stream in a particular plant or refinery. As a result, the PX production is limited, at most, to the amount originally present in the C8 aromatic stream unless additional processing steps are used to increase the amount of PX and/or to improve the PX recovery efficiency. A variety of methods are known to increase the concentration of PX in a C8 aromatics stream. These methods normally involve cycling the stream between a separation step, in which at least part of the PX is recovered to produce a PX-depleted stream, and a xylene isomerization step, in which the PX content of the PX-depleted stream is returned back towards equilibrium concentration.
In a typical aromatics plant, liquid feed, as shown in FIG. 1, typically a C8+ aromatic feedstream which has previously been processed by known methods to remove C7− species (particularly benzene and toluene), is fed by conduit 1 to a xylenes re-run column 3. The xylenes re-run column (or more simply a fractionation column) vaporizes the feed and separates the C8 aromatics into an overhead mixture 5 of xylenes (OX, MX, and PX) and ethylbenzene (EB), and a bottom product 61 comprising C9+ aromatics. The overhead mixture typically has a composition of about 40-50% metaxylene (MX), 15-25% PX, 15-25% OX, and 10-20% EB. Unless otherwise noted herein, percentages are % weight.
The overhead mixture 5 is then to the PX recovery unit 15, which may employ crystallization technology, adsorption technology, or membrane separation technology. These technologies separate PX from its isomers and are capable of producing high purity PX up to 99.9%, which is taken from unit 15 via conduit 17. In the case where unit 15 is an adsorptive separation unit, such as a Parex™ or Eluxyl™ unit, the extract 17, which comprises a desorbent, such as paradiethylbenzene (PDEB), needs to be separated, such as by distillation, from the desired extract PX in distillation column 19. This generates a high purity PX stream 27, which may contain light impurities such as toluene, non-aromatics and water that are removed in a downstream column (not shown) to further improve PX purity. The desorbent is returned to the PX recovery system 15 via conduit 21.
The raffinate 65, which comprises mainly MX, OX, EB, and desorbent is sent to fractionation column 37, generating stream 35, containing MX, OX, and EB, and bottoms 63. The desorbent in the bottoms product 63 is returned to 15. Note that as used herein the term “raffinate” is used to mean the portion recovered from the PX recovery unit 15, whether the technology used is adsorptive separation, crystallization, or membrane. The stream 35 is sent to isomerization unit 43 to isomerize the MX and OX and, optionally, EB to an equilibrium mixture of the four isomers. Isomerization unit 43 may be a vapor phase or liquid phase isomerization unit or both. The product of the isomerization unit 43 is sent via conduit 51 to the C7− distillation tower 53, which separates the product of isomerization into a bottom stream 59 comprising equilibrium xylenes and the overhead 47, comprising C7− aromatics, e.g., benzene and toluene. The bottoms product 59 of distillation tower 53 is then sent to xylenes re-run column 3, either merging with feed 1 as shown in the figure, or it may be introduced by a separate inlet (not shown).
However, the presence of EB in the C8 aromatic streams may impact the efficiency of certain processes described above. In particular, when an adsorptive separation unit is used for PX recovery, while the adsorbent has a higher affinity for PX, it may also adsorb EB to a significant extent, thereby reducing the adsorbent's capacity to adsorb PX. Thus, to avoid EB competing for adsorption capacity and increase the efficiency of PX adsorption, it is desirable to reduce or minimize the amount of EB in the C8 aromatic stream sent to the adsorptive separation unit. Additionally, liquid phase isomerization converts little or none of the EB in the PX-depleted stream, and as a result, the amount of EB in the xylenes loop can build up to very high levels. Thus, to maximize the use of liquid phase isomerization, it is also desirable to control the amount of EB in the PX-depleted stream subjected to liquid phase isomerization.